The objectives are to confirm the hypothesis that late sequelae of human Group A streptococcal (strep) infections are initiated by reactions in the host tissues between strep antigens (Ags) and antibodies (Abs) elaborated by the host, Abs and Ags labeled with fluorescein (Fl), ferritin (Fer) and peroxidase (Po) have been used to localize the immunologic reactants in situ. Data accumulated have shown that in a high percentage of patients with acute glomerulonephritis (AGN), complement and Ag-Ab complexes containing type 12 Ags are lodged in glomerular capillary walls (GCW), particularly in subendothelial and mesangial regions. Plans for following years are: 1) the number of biopsy specimens to be received and examined is expected to increase significantly over the present year. In addition, many transplant (Tx) recipients' "own kidneys," (particularly chronic nephritic), will become available for elution and examination of the specificity of immunoglobulins localized. 2) Immunological findings from human kidney Tx seems best to support the hypothesis that damage to allografts is associated with the deposition, in glomerular and vascular structures, of circulating Ag-Ab complexes some of which may contain transplantation Ags. Antisera to Tx Ags have been received from St. Mary's Hospital, London, and have been conjugated with Fl and Fer for further studies on human allografts. 3) The possibility has emerged that some cases of AGN as well as systemic lupus nephritis (SLEN) may be related to viral infection. This has led to the testing of renal tissues from selected cases of nephropathy with Fl-labeled Abs to various viruses. 4) In rheumatic fever, heart-reactive Abs have been found in high titer in convalescent sera. Such sera will be labeled with Fer for testing on appropriate cardiac tissues to determine the site of localization.